The machining by turning a surface of an ophthalmic lens is also known as a digital surfacing, which is carried out by a turning machine using a machining tool acting in three directions of the machine.
Currently, for machining by turning a surface of an ophthalmic lens, turning parameters are determined and machine defects parameters are determined, independently.
The turning parameters comprises for instance the material of the lens to manufacture, the average curvature or radius of the surface of the lens, the dynamic or the path of the tool, the direction of rotation of the lens (corresponding to the turning axis of the machine) and the cutting data.
The turning parameters and in particular the material, the curvature and the path correspond to data which are given in the order of the lens, while the direction of rotation is determined as a function of the machining tool parameters, and the cutting data are determined as a function of the material or as a function of the dynamic.
The machine defects parameters correspond to offset location values of the tool in the machine and are configured for compensating geometrical defects of the machine, which defects can impact the surface of the lens obtained compared to the surface of the lens targeted.
The machine defects parameters are determined by machining a calibration piece according to a predetermined theoretical geometry by using the machining tool of the turning machine, measuring geometrical characteristics of the calibration piece machined, comparing the data measured with the theoretical data, deducing geometrical defects of the machine and determining the offset location values corresponding to the machine defects parameters.
U.S. Pat. No. 7,440,814 describes a method for auto-calibration of a tool in a single point turning machine used for manufacturing in particular ophthalmic lenses, method in which a test piece of predetermined geometry is cut with the tool and probed to obtain probe data. The method uses the probe data to mathematically and deterministically identify the necessary tool/machine corrections in two directions (X, Y) or three directions (X, Y, Z) of the machine.
In particular, the method first describes a method for calibration of the tool in the X and Y directions (named 2D calibration concept). The method comprises the following steps:                cut a predefined circular groove in a test piece, the groove defining a rotationally symmetrical geometry requiring both positive and negative tool contact angles;        probe the test piece and in particular the curved section line of the circular groove and store the probe data obtained;        execute best fit analysis of probe data to determine best fit of theoretical test piece geometry through the actual geometry of the test piece;        determine X-offset by comparing actual to theoretical results;        determine Y-offset by comparing actual to theoretical results;        execute best fit analysis of probe data to determine best fit circle through a general tool tip geometry;        analyse probe data to determine tool waviness errors in the Y-direction relative to a slope of a tangent angle between tool tip and test piece;        store results of above analyses in appropriate memory register and/or data files; and        use results by appropriately controlling the machine X and Y axes to correct for X and Y axes.        
Next, the method describes a method for calibration of the tool in the X, Y and Z directions (named 3D calibration concept). The method comprises the following steps:                cut a predefined asymmetrical surface along two horizontal axes in a test piece, the surface defining a rotationally asymmetrical geometry;        probe the test piece and store the probe data obtained;        analyse probe data to determine general tool tip geometry, distance from center of best fit tool tip radius to center of lens rotation (in X-direction) and Y-errors relative to slope of tangent angle between the turning tool and the test piece;        probe test piece while rotating it and store probe data;        analyse probe data to determine Z-direction distance of cutting edge of the tool to center of axis of work rotation;        store results of above analyses; and        use results by appropriately controlling the machine X, Y and Z axes to correct for X, Y and Z axes.        
In the method described above, the machine defects parameters are determined only as a function of geometrical characteristics.